I generally try to find 6/4 or 8/4 lumber in planks at least 24" long, 36" if I can, and in widths that are multiples of 2". My risers are 18"x 2 x 1.5. Those sizes let me get multiples, plus leaving some sanding thickness. I can generally find what I want at Rocklers or Cross Cut Hardwoods. You might have to find different hardwood outlets where you are. I make it a practice to inspect all the wood whenever I go in and buy anything good, even if I don't need it right now. If i get a sudden yen to build a particular bow, I have wood on hand.